Back to November 2017

“Son, Nitrogen Makes Plants Grow”

George Ray McEachern’s father, a Louisiana cotton farmer, first told him this years ago, and it’s something he remembers when managing the TAMU orchard.

This week I told Monte Nesbitt, “Well, it looks like we’re going to have two short crops in a row.”

I asked him what he thought. Smiling, he said, “They need a good dose of Ammonium nitrate.” We both laughed because we know one cannot go 15 years without soil-applied nitrogen fertilizer on bearing pecans.

Since 2010 we have been adding 32 percent liquid nitrogen to all our foliar sprays at the Texas A&M orchard at the rate of 1 gallon per 500-gallon tank. This is only 10 pounds of nitrogen per acre per year. Also, we removed 50 percent of all the trees and since then, have received good growth and yields.

However, after six years of good production, we finally had slow growth in 2016 and reduced crop in 2017. The foliar nitrogen spray resulted in a leaf analysis of 2.5 percent nitrate. We had a heavy crop in 2015 and should have applied nitrogen fertilizer to the soil to keep the trees healthy as they filled the crop.

We have a crop this year, but it is not an increase in production. Our long-term goal was to obtain 1,200 pounds per acre each year with no alternate bearing, but that is not going to happen. I am afraid we could have a short crop next year. I am sure there are pecan growers smiling at our struggle and saying, “Welcome to the pecan business.”

Understanding and Determining Nitrogen Application

Many years ago, Larry Stein, Ph.D., in working with James Leo Greer at Rocksprings, Texas learned it is good to apply nitrogen throughout the growing season according to the crop load and tree needs.

The A&M pecan nitrogen recommendations grew from this observation over the following years. One should or may apply up to 100 pounds of actual nitrogen per acre to bearing trees according to the tree’s needs.

If trees had a heavy crop the previous season, 25 pounds of nitrogen need to be applied before bud break. On the other hand, if trees had no crop the previous season, one will need to wait and see if a crop will set in early May. If a crop is set, apply nitrogen in May, June, and July at a rate of 25 pounds of actual nitrogen per acre at each application. These small frequent applications will ensure continued nitrogen availability to the tree, foliage, and developing nuts.

Pecan nitrogen research has been difficult because the volume of nitrogen applied to the soil is not actually absorbed by roots and is lost or unavailable. Pecans may not absorb nitrogen for a number of reasons. Some of which include the possibilities that the nitrogen is leached out of the root zone, volatilized into the air, washed away from the tree with rain or flooding, or even simply in an unavailable form or not in contact with a root.

When we apply 100 pounds of actual nitrogen to the orchard floor, we do not know how much the tree actually absorbs. It is possible that 50, 70, or 90 percent could be lost. Large one-time applications, which are common, can increase the opportunity for absorption but can also stimulate too much growth. On the other hand, applying nitrogen in irrigation water can increase the percentage of absorption and is used by many. Although fertigation in pecans is excellent in flood irrigated orchards, it can limit root distribution to the water pattern for drip-irrigated orchards.

As for roots, they can rapidly absorb nitrogen in the soil, and nitrogen can then move through the entire foliage in a matter of minutes. Because of this rapid absorption, we can feed nitrogen to the soil as the trees need it. We can apply nitrogen when we have a heavy crop and hold off when we have no crop.

A simple or short answer to this dilemma: use nitrogen when the trees need it. Nitrogen applied any time is better than never.

When it comes to determining a tree’s nitrogen levels, first understand that the foliage’s nitrogen level can be excessive, optimum, deficient, or stressed. There are a number of signs for nitrogen needs. One of which is foliage appearance, an excellent indicator of a tree’s nitrogen level. High vigor with deep green color is an indicator of too much nitrogen with reduced flowering in the spring. Large dark green leaves without vigor are perfect.

Over-growth shoots are the extreme sign of too much nitrogen on bearing trees. When bearing pecans produce new growth beyond the nut cluster, the trees have too much nitrogen.

Slow growth is easy to notice. When pecans have a good supply of nitrogen, the shoots grow fast and leaves expand to large size and mature trees will flower and set fruit each year.

Nitrogen should be managed according to soil depth, texture, drainage, pH, and more. Deep, well-drained, loam soil will cover a world of mismanagement; however, shallow or sandy or clay soil can sometimes be almost difficult to manage for top tree health and performance. Good records of nitrogen use and production will tell one what works. Leaf samples from late July indicate the nitrogen level in the foliage.

Look out for these numbers: 2.75 percent is ideal, 2.50 percent is usually ok, and 2.25 percent is too low. Overusing nitrogen will not push the leaf level above 3.00 percent.

Deciphering the Warning Signs

This August, Dr. Larry Stein said, “You don’t have enough foliage.” This was for the entire orchard—all trees, all varieties, all ages. Yes, our trees were not getting enough nitrogen; they had foliage and most had a crop, but there was not enough foliage and it was not dense. Even our trees with no crop did not have a dense leaf crop with dark green color.

Our trees are somewhere between optimum and deficient.

Small leaves on nitrogen-stressed trees are uniformly yellow over the whole leaf. This indicates nitrogen is extremely low or absent. During the growing season, if the lower leaflets are yellow while the apical leaflets are green, a nitrogen shortage exists. Thin shoots also typically signal a nitrogen deficiency.

Premature leaf drop is another indicator that a tree needs more nitrogen.

Besides nitrogen fertilization, other factors can contribute to low nitrogen uptake and less than ideal growth and fruiting. Soil depth, soil drainage, tree crowding, lack of water, lack of zinc, and more can cause low nitrogen uptake and poor foliage.

As for the Texas A&M orchard, we will begin testing various soil nitrogen application systems and rate in 2018. Some of the

We will apply nitrogen to the soil for the entire growing season by using small applications relative to crop size. At the same time, we will attempt to prevent the overuse of nitrogen because excess vigor and compensatory growth can result in alternate bearing. So our goal will be to obtain strong foliage without excessive shoot vigor.

The late Hugo Pape would tell me, we want strong healthy foliage without long shoots on mature trees.

I have worked with orchards that have applied no nitrogen for years but continue to bear pecans at least every other year. These growers look in the top of the trees. If they have good shoot growth and leaves, the tree is not in trouble. However, if there is no growth in the top, if shoots are dead or if leaves and shoots are small in the top; the trees need nitrogen, water, weed control, and zinc foliar sprays. Growers in East Texas or along the Gulf Coast which obtain more than 30 inches of rain frequently do not apply nitrogen to the pecans, but yes, they do fertilize the grass for cattle feed.

Nitrogen management for bearing pecans is complex. The best nitrogen management is both foliar sprays and soil fertilization without excess vigor.

Author Photo

George Ray McEachern

George Ray McEachern is a professor of Horticulture, Texas A&M University, College Station, Texas.